Atmospheric pressure chemical vapor deposition (APCVD) systems are used to form thin, uniform layers or films on a substrate such as a semiconductor wafer. During APCVD processing, silicon dioxide films may be formed by mixing chemical vapors such as tetraethoxysilane (TEOS) with ozone. As the gases are mixed, the resulting chemical reactions deposit a silicon dioxide film on the substrate surface. The desired reactions generally occur in the temperature range of 300.degree. C. to 500.degree. C., with the substrate and chamber being heated to the selected temperature. For semiconductor processing, film characteristics such as moisture content, resistance to cracking, density and the like must meet high quality standards. Another important characteristic is step coverage; that is, the formation of a film of uniform thickness overlying and following the shape of components on the surface of the substrate such as aluminum connection wires and the like. The ability of the deposited film to fill gaps of about 0.18 to 0.25 micron between adjacent surface components is another important factor affecting the quality of the film and integrated circuits formed on the wafer. Obtaining high quality films is becoming of greater significance as the number of layers formed on the substrate increases and integrated circuits become more intricate.
One method employed in APCVD processing is to mix the chemical reactants before they are injected into the reaction chamber to expose the substrate to a more uniform mixture during processing. However, the reaction of the mixed chemicals in the injectors, conduits and the like may result in powder formation which reduces the efficiency of the injector and provides a source of particulate contamination reducing the purity of the deposited film.
With other APCVD processes, the reactants, such as TEOS and ozone, are separately injected into the reaction chamber. With prior art systems, the central port of the injector is typically coupled to the source of TEOS or other chemical reagent while an oxygen/ozone mixture is supplied to two outer ports. Two streams of nitrogen or another inert gas separate the TEOS and ozone to insure that the chemicals are mixed directly above the substrate in the reaction chamber. For a reagent-centered injection configuration, flow-like step coverage of the deposited film is only observed outside of the area immediately below the center port. Thus, the surface conformality in the area of the ozone injectors is typically greater than the area immediately beneath the central TEOS port.
U.S. Pat. No. 5,462,899 discloses a chemical vapor deposition method for forming SiO.sub.2 films in which TEOS is combined with an oxygen/ozone mixture having an ozone concentration of only 0.1 to 1 percent ozone by volume and the mixture is injected into the reaction chamber. A mixture of oxygen/ozone having a higher ozone concentration of 1 to 10 percent ozone is separately injected into the reaction chamber. The TEOS-oxygen/ozone mixture is injected through several ports including the central port.
An APCVD system which optimizes uniformity of the deposited film, providing flow-like step coverage across the entire substrate including the region immediately below the center port, is desirable. An APCVD system in which the deposited film has reduced shrinkage, wet etch ratios, and moisture absorption is also desirable.